The present invention relates to a watermark, and more particularly relates to a technique for embedding watermark information in one-dimensional data such as audio data, and to a technique for extracting watermark information from one-dimensional data in which such a watermark has been embedded.
Pulse code modulation (PCM) is widely used to produce digital audio data. In PCM, an analog audio signal is sampled, quantized, and subjected to linear pulse encoding to produce digital audio data. Music software typically uses PCM involving quantization on 16 bits with a 44.1 kHz sampling rate for digitization. This method can faithfully reproduce sound in the human audible frequency band, so audio can be digitized while maintaining extremely high sound quality.
However, a characteristic of digital audio data is that it can easily be reproduced completely (i.e., can be faithfully reproduced). While the ability to faithfully reproduce audio data is a significant advantage of digitization, it also makes the issue of copyright protection more crucial. In recent years, the so-called xe2x80x9cdigital watermarkxe2x80x9d has been adopted as a countermeasure. Digital watermarking is a technique for embedding watermark information, such as copyright information, in audio data in a form that cannot be detected by the human senses.
However, conventional digital watermark technology has the drawback of significant noise resulting from embedding of watermark information, which, in the case of music software, for example, can significantly degrade sound quality, and is therefore unsuitable where high data quality is required. Another problem is that watermark information is not secure, so that watermark information can be extracted relatively easily by a third party. This problem is not limited to digital watermarking of audio data, but also pertains to digital watermarking of various other types of one-dimensional data such as measurement data, digital control feedback signals, and the like.
The present invention was developed in order to overcome the drawbacks of the prior art, and has as an object to provide a technique affording reduced noise resulting from embedding of watermark information, as well as secure watermark information.
In order to achieve the stated objects at least in part, according to the present invention, in the process of embedding watermark information in one-dimensional source data, first, a spectrum-spreading operation involving an operation equivalent to multiplying the one-dimensional source data by a specific positive/negative value sequence that randomly assumes positive/negative values having a specific absolute value is performed to generate first spread one-dimensional data. This first spread one-dimensional data then undergoes an orthogonal transformation. A specific frequency component value of the transform coefficients obtained through orthogonal transformation is modulated to provide association with the watermark information. The modulated frequency component then undergoes inverse orthogonal transformation to generate second spread one-dimensional data. Then, using the specific positive/negative value sequence mentioned earlier, the second spread one-dimensional data is subjected to spectrum-despreading to produce digitally watermarked one-dimensional data in which watermark data has been embedded.
According to the invention described hereinabove, prior to embedding watermark information, the one-dimensional source data is subjected to spectrum-spreading with a specific positive/negative value sequence, and it is thus extremely difficult to extract the watermark information from the watermarked one-dimensional data without knowing the specific positive/negative value sequence. Accordingly, watermark information can be made more secure by means of spectrum-spreading. Further, according to the present invention, a specific frequency component value of orthogonal transform coefficients is modulated to provided association with the watermark information, thus reducing noise caused by watermark information in one-dimensional data having undergone inverse orthogonal transformation.
In preferred practice, the specific positive/negative value sequence will be generated according to a specific frequency generation key.
This obviates the need to store all specific positive/negative value sequences, since identical spectrum-spreading/despreading processes can be carried out simply by storing the frequency generation key.
The aforementioned orthogonal transformation is a modified discrete cosine transformation wherein each of a number of frames each consisting of 2M (where M is an integer equal to 2 or greater) data from the first spread one-dimensional data undergoes a transformation using a predetermined window function; the first spread one-dimensional data may be divided into a plurality of frames in the modified discrete cosine transformation such that adjacent frames take locations mutually shifted by intervals of M data.
By using this kind of modified discrete cosine transformation, frame distortion can be minimized and noise further reduced.
In preferred practice, the specific frequency component associated with the watermark information will be a frequency component value substantially equivalent to M/2.
In this way, noise can be further reduced.
Embedding of watermark information in a specific frequency component can be accomplished by quantizing a specific frequency component by a predetermined integer kd to generate a first quantized value, associating the least significant bit of the first quantized value with the watermark information bit value through modulation to generate a second quantized value, and multiplying the second quantized value by the predetermined integer kd to generate a modulated frequency component.
Increasing the size of integer kd has the advantage of increasing the likelihood of correct extraction of the watermark information even if a certain level of bit errors should occur in the watermarked one-dimensional data.
According to the present invention, in the process of extracting watermark information form watermarked one-dimensional data in which watermark information has been embedded, first, spectrum-spreading is performed by an operation equivalent to multiplying the watermarked one-dimensional data by a specific positive/negative value sequence that randomly assumes positive/negative values having a specific absolute value, thereby generating first spread watermarked one-dimensional data. This first spread watermarked one-dimensional data then undergoes an orthogonal transformation. Watermark information associated with a specific frequency component value is then extracted from the specific frequency component value of the transform coefficients obtained from the orthogonal transform.
In this way, watermark information embedded by means of the embedding process described above can be extracted from watermarked one-dimensional data.
The several embodiments of the invention are listed below.
(a) A method for embedding watermark information in one-dimensional data.
(b) An apparatus for embedding watermark information in one-dimensional data.
(c) A storage medium for storing a computer program for embedding watermark information in one-dimensional data.
(d) A storage medium for storing digitally watermarked one-dimensional data containing embedded watermark information.
(e) A method for extracting watermark information from digitally watermarked one-dimensional data.
(i) An apparatus for extracting watermark information from digitally watermarked one-dimensional data.
(g) A storage medium for storing a computer program for extracting watermark information from digitally watermarked one-dimensional data.